I have two hearthlings living in the same house, and it makes them slightly unhappy. I wish there was a way to marry them so it doesnt happen. Or at least have a more advanced relationship system that makes it so hearthlings that are friendly with each other are fine with sleeping in the same house.

Yeah, I think some means for hearthlings to build up affinity with each other so that they are happy to share a room together is a good idea. I don’t know if this will take the shape of marriage specifically, but there’s something to be done here for sure.